The subject matter of the present invention relates to a process and apparatus for testing, perforating, and testing an open (not cased) formation traversed by a wellbore, the perforation being required in order to penetrate a low permeability or lowered permeability and/or porosity zone which exists in the region of the formation immediately surrounding the wellbore.
When a wellbore is being drilled, a reservoir filled with oil, natural gas or other valuable fluid may be discovered in the earth's formation. During the drilling, a drilling fluid circulates through the wellbore. If the reservoir is permeable, the formation fluid in the reservoir will enter the wellbore when the reservoir's formation pressure is greater than the hydrostatic pressure of the drilling fluid. In order to prevent blowouts from this effect, the drilling fluid circulates at a pressure which is greater than the particular formation pressure of the formation fluid. As a result, a portion of the drilling fluid, called filtrate, enters the formation traversed by the wellbore and a mudcake forms on the external surface of the wellbore. The mudcake tends to slow the rate at which the filtrate enters the formation; nevertheless, the filtrate continues to enter the formation. When the filtrate enters a formation which contains dry natural gas, the filtrate adheres to the surface of sand grains present near the external surface of the wellbore. Although the sand grains were previously dry, the sand grains are now wet thereby reducing the effective porosity and/or permeability of the formation near the external surface of the wellbore. In another situation, if an oil or gas bearing formation contains a certain amount of shale, when the shale near the external surface of the wellbore becomes wet in the presence of the filtrate, it absorbs the filtrate and expands, thereby reducing the effective porosity and/or permeability of the formation near the external surface of the wellbore. In another situation, fine solid particles created by the drilling process may migrate into the porous material near the surface of the wellbore and reduce the porosity and permeability of such material very near the external surface of the wellbore. When drilling is complete, a drill stem test often commences. During the drill stem test, the pressure existing within the wellbore is less than the pressure of the formation fluid in the reservoir (an underbalanced condition) and formation fluid flows into the drill string. The reduced permeability and/or effective porosity near the external surface of the wellbore tends to restrict or block the flow of the formation fluid from the reservoir into the wellbore (a condition called "skin damage"); as a result, the production capacity of the reservoir cannot be accurately achieved or measured, nor can an adequate sample of the produced fluid be obtained. In this condition, it would be desirable to first eliminate or reduce the skin damage which exists near the external surface of the wellbore in order that a free or natural flow of formation fluid may be produced from the reservoir into the wellbore, and second that the reservoir characteristics may be accurately measured during a drill stem test.
One prior art method of eliminating or reducing the skin damage is to circulate or spot a fluid, such as acid, in the wellbore in order to dissolve the mud or other materials which exist in the formation near the external surface of the wellbore. Another prior art method of eliminating the skin damage is to perforate the formation during the drill stem test. Therefore, one prior art method of performing a drill stem test comprises the steps of eliminating the skin damage by perforating the formation at the moment the test begins while conducting the drill stem test. However, when this method is used, one has no knowledge of the pressure, temperature and flowrate of formation fluids which flow from the reservoir into the wellbore in the presence of the skin damage on the external surface of the wellbore; in addition, when this method is used, the integrity of the packer seat cannot be tested before perforating; and, if little or no skin damage exists, the zone cannot be tested without incurring additional expense, or other possible complications, related to the perforating.